1 Introduction

Recent enhancements at the EVN MkIV data processor at JIVE have boosted our capabilities to offer you the opportunity to map a wider field of view around a single phase center. The principal limitations to wide-field mapping are the smearing effects due to averaging the data in time and/or frequency during the correlation itself (or at some subsequent point). Because there is a finite integration time and a finite frequency-point width, each visibility in your FITS file actually corresponds to an area on the u-v plane: ``radial" extent corresponding to frequency averaging and ``azimuthal" extent to time averaging. Thus points close enough to each other on the u-v plane would not be independently sampled, corresponding to distortions in the map plane far from the phase center. The EVN calculator (www.evlbi.org/cgi-bin/EVNcalc) now incorporates both field-of-view and correlator-capacity computations consistent with the various equations in this paper. Hopefully, you will find this description and these tools help take the mystery out of selecting such parameters when it's time to propose/correlate your experiment.

Here, we'll begin by reviewing the formulae for computing the fields of view resulting from bandwidth and time smearing (§2). Next, we place these purely mathematical results in context for the EVN MkIV correlator by summarizing its current capacity in terms of spectral resolution and output integration time (§3). With these two aspects in hand, we then tabulate the fields of view resulting from various configurations of observation/correlation parameters (§4). We conclude with a few directions for future improvements aimed towards improving the useful field of view for a single correlation phase center.